1. Field of the Invention
This invention relates to a new method of making the electrodes and a tunnel barrier therebetween for a Josephson superconducting device. More particularly, this invention relates to a novel method of making a base electrode, a counter electrode and a barrier junction therebetween as uniformly deposited layers of lead materials over a substrate without removing the substrate from a vacuum chamber.
2. Description of the Prior Art
It is well known that superconducting Josephson junction devices comprise a base electrode and a counter electrode having an insulating layer which separates the electrodes except at a window or aperture in which a tunnel barrier junction is formed. The Josephson junction device is placed in a cryogenic environment and presents a superconducting path for the flow of electrical current until a critical current is reached in the barrier junction. Once the critical current is reached in the tunnel barrier junction area between the electrodes, the barrier becomes highly resistive to the flow of current. It is well known that the application of a magnetic field can be used to control the critical current.
Heretofore, the base electrodes of Josephson junction devices have been made by vacuum depositing niobium and niobium alloys, and lead and lead-alloys on a substrate in a vacuum chamber. In subsequent steps the base electrodes have been removed from the vacuum chamber and provided with photoresist mask to form the window on the base electrode which is surrounded by an insulating layer such as silicon oxide. Subsequent steps require that the mask material be removed from the area where the tunnel barrier junction is to be made before the barrier material is deposited. Once the barrier area is exposed, the barrier junction may be made by depositing the barrier material or oxidizing the area in the window. During deposition of the barrier contamination of the tunnel barrier junction proper may result from the surrounding insulating material. The barrier area may also be contaminated during the process of chemically applying or removing the photoresist mask.
To avoid this contamination of the tunnel barrier junction, it has been suggested that the base electrode layer, the tunnel barrier junction and the counter electrode could be deposited successively in the same vacuum chamber before the substrate is removed. When the three aforementioned layers are deposited successively without being removed from the vacuum chamber, they would then form a laminate or sandwich which was not subject to impurities. Attempts to remove portions of the sandwich by etching or milling have not proven successful. Chemical etching of the upper electrode cannot be controlled precisely and will destroy part of the base electrode. Ion beam milling can be controlled more precisely, but is still not accurate enough to mill away the upper layers and delineate the barrier junction without attacking the base electrode. It is known that ion beam milling of a layer will redeposit material on the side wall where the cut is made. Thus, ion beam milling and plasma etching etc. can redeposit superconductive material on the side wall and cause shorts. Both chemical etching and the aforementioned milling processes can cause shorts and undercuts at the tunnel barrier junction.
Copending application Ser. No. 179,331 filed Aug. 18, 1980 and entitled "Josephson Tunnel Junction Device and Method of Manufacture" by Harry Kroger is assigned to Sperry Corporation (the same assignee as the present invention assignee). This copending application teaches a method of making Josephson superconducting devices employing niobium and/or niobium nitride. It is well known that niobium and niobium nitride are anodizable materials. Portions of the counter electrode made of niobium may be changed from a superconducting material to a resistive material by anodization or oxidation of the niobium counter electrode layer after it is deposited. This technique will result in delineating the tunnel barrier junction by leaving a portion of the counter electrode unanodized. The aforementioned method and structure taught in this copending application are not applicable to the manufacture of lead alloy Josephson junction superconducting devices because lead and lead-alloys cannot be anodized to produce a resistive lead material.
Accordingly, it would be desirable to produce lead and lead-alloy electrodes for Josephson junction superconducting devices with a tunnel barrier junction therebetween in a vacuum chamber environment without having to remove the device from the vacuum chamber. This would permit the manufacture of the two electrodes and the tunnel barrier junction without being subject to the introduction of impurities.